The present invention relates to the field of deploying, operating, and recovering a tethered submersible vessel from a ship.
Submersibles are launched from ships by a variety of vehicle handling systems utilizing cranes, lines, and tethers. Tethered vehicle handling systems are classified into two types: submersible only, and submersible with support cage. Some systems use the tether for lowering the submersible; others use a separate line.
The submersible only system generally consists of a neutrally buoyant vehicle and a tether cable coupled to a support platform such as the deck of a ship. A section of the tether near the submersible is rendered neutrally buoyant with floatation collars in order to decouple transmission of tensile forces through the tether due to relative motion between the ship and the submersible. Standard procedure requires the deploying ship to stop during launch and recovery of the submersible. Immediately after launch, a submersible deployed with this type of system floats near the support ship. In an adverse sea or wind, the submersible is easily pulled beneath the deploying ship by propwash or suction as the ship heaves. Shock loads induced in the tether due to relative motion between the ship and submersible may be high, even if a separate lift line is used to support the weight of the submersible during launch and recovery operations. If the submersible is supported by the tether cable during launch and recovery, relative horizontal motion between the submersible and the support ship will cause the vehicle to be towed. Since the tether cable generally exits the center top of the vehicle in this configuration, the vehicle will be rolled over, causing high bending loads to be applied to the cable termination point.
The vehicle/cage combination system was developed to overcome the launch and recovery problems of the submersible only system at the sea surface. With the vehicle/cage system, the neutrally buoyant vehicle is mated to a heavy support cage. The cage and vehicle may easily plunge through the sea surface and be lowered to operating depth where the vehicle unmates from the cage. The vehicle swims off as a neutrally buoyant secondary cable is fed from the cage. There are several inherent problems with this approach which are moderate for shallow systems and severe for deep diving ones. Due to ship induced motion of the cage, the cage or vehicle can be easily damaged during recovery operations when the cage and vehicle are re-mated. If mating cannot be accomplished, the recovery of the separate cage and vehicle without damaging the secondary cable becomes very difficult.
The problem of relative vertical motion between the cage and the vehicle can be solved with difficulty by providing motion compensation at the surface. Except for stretch in the cable, the cage follows the motion of the ship. Therefore, motion compensation is necessary so that there is no relative motion between the cage and the vehicle. Motion compensation is accomplished in two basic ways: Actively, using a feedback system to drive actuators to eliminate the relative motion between the vehicle and cage, and passively, using actuators supported by a pneumatic system that acts as a very soft shock absorber. The actuators generally drive a crane or A-frame boom (boom bobber) which supports the cable. A ram tensioner including ram sheaves through which the tether is reeved can also be used, but is generally undesirable because of fatigue effects on the tether. Both of these systems use hydraulic cylinders to drive the crane that supports the tether cable in an effort to eliminate the vertical motion of the end of the crane. Both systems are costly to build and maintain due to their complexity. Motion compensation may also be required for vehicle/cage systems in order to avoid cable resonance during vehicle excursions 10,000 feet below the ocean surface.
An additional problem with deep diving vehicle/cage systems is not generally recognized: as operating depths become deeper, the vehicle and cage components become larger and heavier. Hence, the strength of the secondary cable relative to the vehicle mass diminishes. Thus, if the vehicle and cage drift apart, any resulting snap load can easily damage the secondary cable. Although the vehicle/cage approach greatly simplifies surface handling, it results in a second submersible, complex surface motion compensation equipment, and a complex submerged docking operation.
Thus, a need exists for a shipboard vehicle handling system that can launch and retrieve submersible vehicles that avoids the complexity and difficult underwater handling of the vehicle/cage systems, and the deployment and recovery problems of the submersible only system.